Means for making impedance measurements at high radio frequencies



Feb. 20, 1940.

yP. S. CARTER Filed March 26, 1938 INVENTOR. P /LP 5. CARTER BY www.

ATTORNEY.

atente Feb. 20,

MEANS FOR' MAKING IMPEDANCE MEAS- UREMENTS AT HIGH RADIO FREQUEN- CIES Philip S. Carter, Port Jeiferson, N. Y., assigner to Radio Corporation of America, a corporation of Delaware' Application March 26, 1938, Serial No. 198,200

- 6 claims. .(cl. 1v1- 95) The present invention relates toa simplied and improved'apparatus for'measuring the reactive and resistive components of an impedance at high radio frequencies.

In the high frequency/"artl it is important to know the values of resistance and reactance, inductiv or capacitive, in an impedance before placing it into service in a circuit. At frequencies higher than ten megacycles, the measuring 10 meters ordinarily` available cannot be depended upon for reading absolute values of current or voltage; in fact, at frequencies higher than thirty megacycles, meter readings are. ordinarily `extremely unreliable. It is known that ar resistor l5 unit having the properties of a practically pure resistance at low lfrequencies may become an impedance consisting of some entirely diierent value oi resistance and a large inductive or capacity reactance at very high frequencies.

-20 The present invention enables the accurate measurement of any type of impedance, and its resistive and reactive components, under the exact condition of frequency, type of mounting and connection leads under which it is to be used. By means of a simple procedure, according to the inventiom-I entirely avoid many doubtful factors present in methods commonly used heretofore in the measurement of impedances.l

In general, the invention makes use of a radio frequency transmission line, either closed or open at its far end, having a length equal to or greater than three-quarters of a wavelength, connected to a high frequency source; a'pair of terminals for connecting the unknown impedance to be measured across the transmission line at a location according to a definite law to be described later;

a sliding radio frequency metereither inductively or electrostatically coupled `to the wires; and the necessary equipment for measurement of distance along the line.

One advantage of the present invention is that the values of the resistive and reactive terms of any unknown impedance may be accurately deter-f mined without the use of ammeters and volttaken in conjunction with the Flgs.- 1 and 2 of the invention, showing schematic diagrams of two preferred arrangements of the'invention.u

Referring to Fig. l, an arrangement for making measurements in accordance lwith the invention I comprises a suitable' source of high frequency energy l, which is connected to one end of a transmission line 2, terminating at the other end in short circuiting bar orstrip 3. Transmission line 2 has a lengthequal to or greater than threequarters of a wavelength (SAA) at the operating frequency. Numeral -4 represents the unknownA impedance which is connected across the line at the connection points 5 which are located in a manner described in detail hereinafter. K For determining the location of the terminating impedance 4 and the ratios of maximum to mini` mum current, or voltage, there is provided a sliding galvanometer 6 in series with a`wire loop 1; both galvanometer and loop being mounted on a support 9, shown in dotted lines, in a manner such that the'locp and galvanometer may be slid .along the wires of the transmission line 2 without varying their relative positions to the conductors of the line. A measuring scale 8 is also shownpositioned along theline for convenience in accuratelylocating the positions of the sliding meter 6.' Maxima of current occur on the line at points spacedk one-half wavelength apart. Similarly, minima also appear at points on the line spaced one-half wavelength apart, the minima points being located a quarter wavelength from the maxima points. By sliding support 9 along the wires of line 2, these points of maxima and minima will be indicated in the meter 6. It 'will be obvious from a mere inspection of the drawing, that the meter is inductively coupled to the line by means of the wire loop 1. Y

The manner in which the lvalues of the resistive and reactive components of the impedance 4 is obtained (by'means of the arrangement Vof Fig. 1) will now be described:

The power source I is `iirst loosely coupled to the two wire line 2 at its input end, appearing at the left of the line. Before placing the impedance 4 in shunt across the lineya'nd without any other impedance shunti'ng the line, the positionof the current node nearest to the short circuiting bar or strip 3 is accurately located. The position will be found to be )J4-q, where q is the end correction for the short circuiting strip. It will be found that the correction q has a value be-A tween zero and S/2 wheres/2 is the conductor spacing. Thisis the position for connection of the' unknown impedance 4. The scale 8 should u be laid below the line 2 with. its zero reading' at this currentnode point. l

The impedance!A to be lmeasured should now be connected across'the line at the current nodal point nearest the strip 3, and the ratiofof maxi- I mum to minimum currentthen obtained from the sliding meter. Assume this ratio to be Q. The position of the current minimum on the scale is now accurately determined.` lThis can be most accurately done by taking the vmid position between two equal readings of the meter, one each s side of the'mim'mum, where the rate of change of current withV position is greatest.. Call the distance from the impedance to thei'oregoing current d. Y

The value of Z is then; .y

2Q+J'(Q2 l) sinv (720.d/A) Q2-i- 1 (QL l) cos (720.d/ l.) the resistance and reactance terms being given by the real and imaginaryparts of the above expression. When the reactance is inductive, the

sign Vof the imaginary term is positive, and whenA Z-o 120 loge olims f When determining the reactance of an impedance consisting of a coil or condenser having negligible losses,` the ratio Q becomes extremely high and need not be measured. The reactance X is then given in terms of the distance by the relation:

X=z0 conl (scura/i) ohms In order that the presentV invention may be better understood, atheoretical explanation will now be given.

When asinusoidal voltage, represented bythe real part of'Eeiwt, is impressed upon any` transmission line, the voltage and current at any position X distance from the load are given by:

I =EI1` {Ie-ff (2) where En is the main wave voltage, Zo the surge impedance, X the distance from the load end.

and

' the reflection coefcient,

W2K', j/, .,`=2f

and e=`Naperian base.

These may be written inthe form:

, The impedance at `Janv point ofthe line looking toward the load is;

when X is the distance to the minimum current position. Let particular value of X be d, then I im. Q-l Q I min. and l Z Z 2Q+j(.Q2-1) sin 21nd oQi-l- 1 (QL- 1) cos 21nd 'I'he last formula corresponds to that given above for determining the value of impedance 4.

It will be understood, of course, that the transmission line, if desired, may be open-ended and the impedance to be measured connected at the open end, after which the ratio Q is obtained, and the same procedure as before followed to determine the value of the impedance. In the arrangement described above, the end eiect of an open line upon measurement is eliminated. 'This is due to spreading of the eld from the end causing a capacity. In the case of the openended line, however, there is asmall capacity eiect in parallel with the impedance due vto line end effect, but usually this is negligible.

When the resistance component of a load im#` pedance is either greater than ten times or less than one-tenth the value of the surge impedance,

it is dicult to accurately determine the ratio Q of maximum to minimum current or voltage.

'Ihis diiilculty may be overcome -by the following procedure:

Increase'either the power or meter sensitivity or botli. until the minimum .current 4or voltage reading is of suiilcient value to be accurately read on the meter. Now choose any arbitrary larger value than the reading and slide the meter both toward and away from the.A

, 1" v @kme-H l (16)". 'when =360Xs/X. (17) I n place of the closed loop with sliding meter, awirewiththemeteratitscentermaybe used, the wire's ends being capacity coupled to the line conductors through a small spacing. -In this case the meter will read voltage distri-- bution, and the fact that a voltage loop coin-4 cides with a` current minimum should be taken into consideration in applying the relations` given. These are easily modiiled to give the impedance values `in terms of distance to a voltage node and voltage ratio.

Fig. 2 shows an arrangement -in accordance with the invention for making measurementson a concentric line. 'A suitable source of high irequency energy I' is connected to one end of a two wire transmissionline Il which terminates in a coupling loop Il. This coupling loop Il furnishes high frequency energy to the concentric transmission line 2'. This concentric line is electrically closed at both ends by the end plates 3', 3".' The unknown impedance is represented by l' which is connected between the inside and outside conductors at the connection points 6.. The connection points B' are located at a distance from the far end terminating plate 3" which is equal to one-quarter wavelength. llorl determining the location and the ratios of maximum to minimum voltage there is provided a vacuum tube voltmeter 6' connected through a shielded cable I2 to a suitable probe 1' mounted on a support 9', in a manner such that the probe may be slid along a slot cut in the outer conductor of the concentric line. A measuring scale 8 is also shown in position along the line for convenience in accurately locating the position of the sliding probe 'l'.

The method of using the arrangement of Fig.

' 2 to measure'the resistive and reactive components of the impedance 4 will now bedescribed. As already mentioned, the impedance l' is connected to the position exactly one-quarter wavelength from the short circuited end of the concentric line. It has been found both from theoretical considerations and actual measurement that the voltage maximum or current nodal point always exists at exactly one-quarter wavelength from the closed end of a concentric line. I n other words, there is no end correction such as is encountered with an open t o-wire line. Since the meter shown in this arr ement reads voltage, this must be taken into account whendetermining the values of the components ot the unknown impedance 4'. Otherwise the procedure is similar to that already described in connection with the use of the arrangement of Fig. l. If We call the distance from the impedance l' to a voltage maximum s, todistinguish this distance from that for a. current minimum which' we have designated asd, we have the relation s=`diA/4. In terms of the distance s and the known impedance ratio Q ot maximum to minimum voltage, ,the resistance and reactance components of the unl' are obtained from the lformula;

ohmsl 'Die surge impedance for the lconcentric line is given bythe formula Zo=60 log+b/a where b and a are the radii of the outer and inner con- 'ductors respectively. When determining the reactance of an impedance consisting of a coil or condenser having negligible losses, the ratio 1Q becomes extremely high and need not be measured; The reactance is then given in terms of -t'hedirltarice' s by the relationr` l X=Zo cot (360 s/A) ohms (i9) What is claimed is: j 1. In a system for electrical measurement, the

combination of a pair of parallel conductors, a source of high frequency energy coupled to said conductors at one end thereof. an impedance to be measured connected across said conductors at the current nodalV point nearest the other end of said conductors, a' sliding meter coupled to said conductors between said impedance and said source, said meter being arranged for movement along the length of said conductors, and means adjacent said conductors for determining the position ofcurrent or voltage maxima or minimay on said'line with respect to the location of said I.

impedancia.'

2. In a system for electrical measurement, the

combination of a pair-oi?4 parallel conductors, a

source of high frequency energy coupled to said conductors at one end thereof. an impedance to be measured connected across` said conductors at the current nodal point nearest the other end of said conductors, a sliding meter coupled to said conductors vbetween said impedance and said source, said meter being arranged for movement along the length of said conductors and having a loop of wire arranged between said parallel"con ductors for inductive coupling thereto, and means for determining the position of energy maxima or minimaI on said line with respect to the loca,-

tion otsaid impedance.

3. In a system for electrical measurement, the combination of a pair of parallel conductors, a

` source of 'high frequency energy coupled to said conductors at one end-thereof, a short-circuiting strip connected across the other end of said con- Y ductors, an impedance to be measured connected across said conductors at the current nodal point l -and means adjacent said conductors for determining the position of current or voltage maxima or minima on said line with respect to the location of said impedance.

4. In a system for electrical measurement, the combination of a concentric line whose inner and outer conductors are electrically closed at both ends, a source of high frequency energy coupled to said conductors near one end thereof, an impedanceto be measured connected across the inner and outer conductors of said line at a. distance equal to one-quarter'wavelength from the other end of said conductors, a meter coupled to said conductors between said impedance and v said source, means for moving along a portion of the length of said conductors the eilective coupling point oi said meter to said conductors, and a scale the other end of said conductors, a vacuum tube voltmeter connected to a probe, said probe being mounted on a movable support on said line and being movable 015er a. portion of the length of said line, said portion of line having a slot in the outer conductor for enabling the probe to be inserted into the interior of said line, and a scale positioned along said portion of the line for,

accurately locating the position of said probe.

6. --A system in accordance with claim. 1, in which said pair of parallel conductors are longer than three-quarters of the length of the operl.

ating wave. y

PHILIP S. CARTER. 

